1. Field of the Invention
The present invention relates in general to an image sensor, and more particularly to signal charge transfer devices of an image sensor and a method of making the same.
2. Description of the Prior Art
An image sensor may generally be classified into a charge coupled device (CCD) image sensor and a contact image sensor. The image sensor receives an optical signal corresponding to an image and converts the received optical signal into an electrical signal. The image sensor then processes the electrical signal to be displayable or printable. A signal charge transfer device is typically provided as an essential component in the CCD image sensor or the contact image sensor, to transfer the electrical signal in a single direction.
Signal charge transfer devices of the CCD image sensor will hereinafter be described as an example of the signal charge transfer device according to the prior art.
The CCD image sensor essentially comprises a semi-conductor substrate such as a silicone, a plurality of optical detectors, vertical signal charge transfer devices and a horizontal signal charge transfer device, all of which are formed on the semi-conductor substrate. A P-N junction photodiode is generally used as the optical detector. Selecting a proper device as the optical detector makes it possible shooting of an object from a visible ray region to an infrared ray region.
Referring to FIG. 1, there is shown a schematic block diagram of a conventional CCD image sensor of an interline transfer type. As shown in this drawing, the conventional CCD image sensor comprises a plurality of uniformly arranged photodiodes 1 , vertical signal charge transfer devices 2 arranged between adjacent columns of the photodiodes 1 , a horizontal signal charge transfer device 3 arranged perpendicularly to outputs of the vertical signal charge transfer devices 2, and a sensing amplifier 4 for sensing signal charges outputted from the horizontal signal charge transfer device 3, amplifying the sensed signal charges by a predetermined amplification degree and outputting the amplified signal charges as voltage information of desired gain.
In FIG. 1 , the signal charges are transferred in directions designated by the dotted arrows.
The operation of the conventional CCD image sensor with the above-mentioned construction will be described in brief.
When light is incident on the photodiodes 1 corresponding to pixels of a frame, the image signal charges are produced in the photodiodes 1. The image signal charges from the photodiodes 1 are transferred to the vertical signal charge transfer devices 2 and then to the horizontal signal charge transfer device 3 upon application of predetermined clock signals to electrodes (not shown in FIG. 1) which are formed over the photodiodes 1 and the vertical signal charge transfer devices 2.
The image signal charges transferred to the horizontal signal charge transfer device 3 are then transferred toward the output of the horizontal signal charge transfer device 3 upon application of predetermined clock signals to electrodes (not shown in FIG. 1 ) which are formed on the horizontal signal charge transfer device 3. In the horizontal signal charge transfer device 3, the image signal charges are transferred through an output gate electrode (not shown in FIG. 1) to a floating diffusion region (not shown in FIG. 1), in which the transferred image signal charges are stored temporarily. Thereafter, the image signal charges stored in the floating diffusion region are sensed by the sensing amplifier 4, which amplifies the sensed image signal charges by a predetermined amplification degree and outputs the amplified image signal charges as voltage information.
Referring to FIG. 2, there is shown a sectional view of the horizontal signal charge transfer device 3 taken on the line a--a' of FIG. 1. As shown in this figure, the horizontal signal charge transfer device 3 comprises a first conductive type or n.sup..cndot. type semi-conductor substrate 5, a second conductive type or P type well 6 formed on the n.sup..cndot. type substrate 5, a n.sup..cndot. type channel region 7 formed on the P type well 6, a n+ type floating diffusion region 8 formed on the P type well 6, being connected to an end portion of the n.sup..cndot. type channel region 7, for temporarily storing the image signal charges from the n.sup..cndot. type channel region 7, a n.sup..cndot. type reset region 9 formed on the P type well 6, being connected to an end portion of the n+ type floating diffusion region 8, a n+ type reset drain region 10 formed on the P type well 6, being connected to an end portion of the n.sup..cndot. type reset region 9, for discharging the image signal charges transferred from the n+ type floating diffusion region 8 through the n.sup..cndot. type reset region 9, a plurality of uniformly spaced n- type impurity regions 11 formed on the n.sup..cndot. type channel region 7, a plurality of first electrodes 12 formed on the n- type impurity regions 11, a plurality of second electrodes 13, each formed between adjacent ones of the first electrodes 12, an output gate electrode 14 formed over the end portion of the n.sup..cndot. type channel region 7, for receiving a constant direct current (DC) voltage, and a reset electrode 15 formed over the n.sup..cndot. type reset region 9, for receiving a reset clock signal R.phi.. The first and second electrodes 12 and 13 alternately arranged constitute a plurality of pairs including odd pairs to which a first clock signal H.phi.1 is applied and even pairs to which a second clock signal H.phi.2 of the opposite level to that of the first clock signal is applied.
Herein, concentrations of n type impurity ions are represented by n.sup..cndot., n+ and n- and levels thereof can be expressed in the following order: EQU n+&gt;n.cndot.&gt;n- (1)
Although not shown in FIG. 2, the sensing amplifier 4 in FIG. 1 senses the image signal charges temporarily stored in the n+ type floating diffusion region 8 in the horizontal signal charge transfer device 3 and amplifies the sensed image signal charges by the predetermined amplification degree.
In FIG. 2, the reference numeral 16, not described, is an insulating layer. Also, a polysilicon doped with an impurity may typically be used as materials of the first and second electrodes.
The operation of the horizontal signal charge transfer device 3 with the construction mentioned above with reference to FIG. 2 will hereinafter be described in conjunction with FIGS. 3a to 3c, which are potential diagrams of the horizontal signal charge transfer device 3 which are different according to states of the clock signal s H.phi.1 and H.phi.2 being applied thereto.
Upon no application of the first and second clock signals H.phi.1 and H.phi.2 to the first and second electrodes 12 and 13 under the condition that the image signal charges from the vertical signal charge transfer devices 2 are transferred to the horizontal signal charge transfer device 3, the potential diagram of the horizontal signal charge transfer device 3 is as shown in FIG. 3a.
Upon application of the first clock signal H.phi.1 of high level voltage and the second clock signal H.phi.2 of low level voltage to the first and second electrodes 12 and 13, the potential diagram of the horizontal signal charge transfer device 3 is as shown in FIG. 3b.
Also upon application of the first clock signal H.phi.1 of low level voltage and the second clock signal H.phi.2 of high level voltage to the first and second electrodes 12 and 13, the potential diagram of the horizontal signal charge transfer device 3 is as shown in FIG. 3c.
Under the condition that the first and second clock signals H.phi.1 and H.phi.2 are applied to the first and second electrodes 12 and 13, with the high and low states being repeated, the image signal charges are transferred toward the output gate electrode 14. At this time, since the constant DC voltage is being biased to the output gate electrode 14 in the horizontal signal charge transfer device 3, a potential is constantly formed in the region under the output gate electrode 14, as shown in FIGS. 3a to 3c, Under this condition, upon application of the first clock signal H.phi.1 of high level voltage and the second clock signal H.phi.2 of low level voltage to the first and second electrodes 12 and 13, the potentials in the regions under the last ones of the first and second electrodes 12 and 13 become high, thereby causing the image signal charges to get beyond the potential barrier in the region under the output gate electrode 14 and then to flow to the floating diffusion region 8. This case is shown in FIG. 3b.
On the contrary, upon application of the first clock signal H.phi.1 of low level voltage and the second clock signal H.phi.2 of high level voltage to the first and second electrodes 12 and 13, the potentials in the regions under the last ones of the first and second electrodes 12 and 13 become low, thereby causing the image signal charges not to get beyond the potential barrier in the region under the output gate electrode 14. As a result, no image signal charge is transferred to the floating diffusion region 8. This case is shown in FIG. 3c.
Herein, the n- type impurity regions 11 may be formed by implanting a second conductive type or P type impurity such as a boron in the corresponding regions of the n.sup..cndot. type channel region 7. Namely, implanting the boron ions in the n.sup..cndot. type channel region 7 causes the n.sup..cndot. type concentration of the implanted regions to be lowered to n- type.
As shown in FIGS. 3a to 3c, the image signal charges are transferred toward the output of the horizontal signal charge transfer device 3 by the stepped potentials which are formed by the n- type impurity regions 11.
The two phase clocking method as shown in FIGS. 2 and 3 may typically be employed as the clocking method of the horizontal signal charge transfer device 3.
Although not shown, a construction of the vertical signal charge transfer device 2 is substantially the same as that of the horizontal signal charge transfer device 3.
Referred to as a signal charge transfer efficiency (CTE) is a degree which the image signal charges are transferred efficiently with no loss in the vertical and horizontal signal charge transfer devices. The image sensor of good quality has higher CTE. The CTE is in inverse proportion to a length 1 of the electrode.
However, the conventional image sensor has a disadvantage, in that, when the number of pixels and a size of an optical system are determined, a length L of a horizontal unit cell is determined and the length l of the electrode cannot be lowered below L/4. This results in a limitation in increase of the CTE.